The invention relates generally to high-tension ropes and, more specifically, to stress-member ropes used in head and tail lead-in sections of instrumented ocean-bottom cables.
Ocean-bottom cables (OBC's) instrumented with hydrophones, geophones, accelerometers, and other sensors are used in seismic prospecting, especially in relatively shallow waters. The cables are laid on the sea floor in a pattern over a survey area. The sensors respond to reflections of seismic signals off geologic structures below the sea floor in the survey zone and other seismic disturbances. The OBC's have active sections, in which the sensors reside, separated by isolation sections. The isolation sections dampen acoustic noise and interference that can propagate along the cable. Isolation sections at the ends of the OBC are referred to as lead-in sections. The lead-in isolation sections include two connectors: (a) a nose cone that attaches to a tow or buoy cable; and (b) a housing penetrator that attaches to an active section. A rope runs back and forth between the housing penetrator and the nose cone around bollards on the peripheries of each. The internal isolation sections have a similar rope-bollard arrangement. Using a rope, which compresses, instead of a steel cable, which does not, as a stress member provides acoustic isolation. When the OBC is being deployed or retrieved or when wave action is causing attached buoys to move about, the ropes in the lead-ins especially are subjected to high levels of tension and to torsion about the bollards. Friction caused by the rubbing of the ropes on the bollards can cause the ropes to fray and, unless replaced, eventually to break. Once the rope breaks, that end of the OBC is separated from its buoy or from the cable-laying vessel. In a worst-case scenario, the instrumented OBC is unretrievable and lost.
Thus, there is a need for an OBC lead-in rope that has a longer lifetime.